Where can one acquire small (20-30) qty magnets suitable for use in
Hall effect tach type applications? I'm thinking of the ultra-slim
magnet splinters you see on floppy drive spindles and VCR head drums,
about 2mmx1mmx0.5mm. After numerous attempts on my junkpile motors, I
have not yet succeeded in salvaging one intact :)

In a related question, what are these normally attached with? I'm
experimenting with motors with a max no-load speed of just under
5000rpm, under temperature conditions expected to range from -10 to
+50 Celsius. I'm thinking an epoxy resin is probably best, but I'd be
curious to hear what other people use.

Since you just need splinters, why not shatter a larger magnet (steal
one from the fridge...) by brute force? Shouldn't be too hard to do,
given a solid surface and a sledge hammer ;->

I'm not at all certain, but IMHO epoxy and cyan-acrylate
("super-glue") should both work out fine. Preferrably avoid using
very much of either, ie have the mounting place fit the shape of the
magnet as closely as possible.

Which glue is better would depend on what you want to glue it on:
super-glue works best between two brittle, inflexible items, becaust
it's quite brittle itself. Epox is somewhat more flexible.

--
Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

I have tried cutting a very thin piece off a flexible fridge magnet,
and also tried to saw a piece off a neodymium magnet out of a hard
disk, but the resulting speck is not strong enough to actuate the Hall
effect sensor. So I think I need to buy it pre-manufactured in the
right size.

Yeah, this is why I wanted to use the flexible magnet :) If it falls
off, the system reverts to an open-loop control system, and this is
going to lead to big inaccuracies. The principal application is
direct-drive of a boat propeller. I have no way of measuring the speed
of the boat directly most of the time (because it is submerged, can't
use GPS), and I found that PWM duty cycle vs. speed of vehicle isn't
well correlated (because there are other variables, esp. water depth).

This is Usenet so, Without knowing any of the details of your
application, I'll leap in with something that's probably obviously not
workable if one's actually holding the application, but ...

Why not an LED/photo-transistor detector pair? I'd imagine that painting
alternating white and black areas around the shaft circumference would
be a whole lot simpler than reliably mounting tiny magnetic chips.

Or, if a Hall effect really is indicated, tear apart a muffin fan of the
appropriate diameter (if it's a small fan and your app is a large
motor). I sacrificed one recently (broken blade) to salvage the Hall
sensor and found that the permanent magnet on the rotor was nothing more
than a "refrigerator-style" magnet molded into a circle to fit inside
the hub. You would have only two poles but would gain a much larger
surface area for the adhesive.

There are people on www.ebay.com that sel the small 1/16" and 1/8"
cylindrical or square magnets from time to time.
That is where I got the ones I use.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item29%76314010&category14%69
Also some of the small flexible magnets they use for advertising andsticking
to things like the refridgerators, may work OK.
You can cut these with a pair of sissors into small peices and stack them up
even. But some of the hall effect sensors may not be sinsitive enough for
these.

My application is also outdoor, and underwater :) In other words, I'm
close to testing a "non-flight-qualified" version of my submarine.
I've tested parts of it, with mostly satisfactory results, but I still
have yet to solve the problem of preventing water ingress at the
propeller and plane glands. The electronics and software are really
simple compared to these horrid (and EXPENSIVE) mechanical engineering
problems. I doubt I will have a fully demonstrable prototype by the
time my book has to hit the publisher, but at least the subsystems
work individually [more or less ;)] and it all works together on the
breadboard.

With New York the way it is right now (you can see the Empire State
Building from my back porch, which means I have military aircraft and
police helicopters buzzing around all the time), it seems an
inopportune time for field tests, anyway.

Could you accept that some leakage is inevitable and install a bilge
pump system? A small amount of leakage may even be desirable in that it
will help to lubricate the shaft packing. Perhaps a small shield just
inboard of the gland(s) to collect the leakage and pipe it to a bilge
tank. Adds another variable to the buoyancy issue, of course, but
designing to be leak-tollerant up to some defined rate (depending on
depth and pump capacity) may be a Good Thing.

I do accept that some leakage is inevitable, and the electronics is on
a raised platform to keep it dry as long as possible. I'm getting more
leakage than I can deal with, though. So far the best solution I can
come up with is to run the prop shaft through a piece of metal tube of
just slightly larger bore than the shaft, and pack the space with
viscous grease. It doesn't work very well.

Due to expense and danger (I had a couple of narrow escapes from being
shot by pieces of bursting fittings), I had to abandon my original
plan for active depth control, which means I don't have an internal
source of compressed air - so there's only so much pumping I can do.

I can't help but think that this problem was
solved about 80 years ago, and not by a bunch
of EE's (:

I am not familiar with propeller shafts, but
I have seen the packing on old steam engines.
They generally have something called a stuffing
box or a packing gland, where grease-covered
flax or cotton string is forced between the shaft and
the box, sealing the joint but still allowing
shaft motion.

I thought about it, but they're really expensive. Also, I need stepper
motors as well as straight DC motors - search for "waterproof stepper
motor" and see just how incredibly expensive they are!

Besides, this engineering problem has solutions that work - I just
haven't found one that lies within my mechanical skills yet.

I have here on my desk some sealed bearings that, if mated with an
exactly fitting shaft, perform very effective water exclusion up to
2atm, but unfortunately I need 3atm - and I would prefer to have an
actual "rated for xxxkPa" part rather than just testing some scrounged
parts. Oh, well. I'm just about to order the "chassis" (skeleton) for
the preflight version, I don't have to make final decisions on all of
this until a little later - there's plenty of room to add extra
sealing hardware, or to bolt on outboard motors, if necessary.

I've seen a drawing of a sealed mechanism to transfer rotary power
through a diaphram (sheet of rubber, such as from a bicycle inner
tube). The shaft going through it doesn't turn (and is thus perfectly
sealed), but moves along the surface of a cone with the apex at the
diaphram. A standard bearing on each side converts the motion to that
of a regular turning shaft.
Another approach is magnetic coupling. Torque can be transferred by
a rotating magnet on each side of a sealing non-magnetic plate. You
can get small, powerful Neodynm (SP?) magnets out of old hard disk
drives as well as the usual sources such as ebay.

It seems like the things I described above should be available
somewhere as off-the-shelf parts. I presume you have a Grainger and
Mcmaster-Carr catalogs, which should have that sort of thing.

In this part of the world attempting to acquire nitric or muriatic
(hydrochloric) acid is likely to bring down the Ashcroft/Bushian
forces and result in incarceration in the Gitmo archipelego with
no further communication with the outside world, lawyers, etc.

Last time I was in the building supply store, they still sold
gallon jugs of muriatic acid without an ID check. It may
not be reagant grade, but it is strong enough to etch concrete
very rapidly (its intended purpose).

Getting nitric acid of a purity and strength suitable for
the manufacture of nitrate-based explosives is probably
more difficult. None of the procurement problems are
anything that couldn't be solved by an attentive student
of a college freshman course in chemistry. And you can
get the same education in a lot of places without professors.